1. Field of the Invention
The field of the present invention is bone repair and replacement. More specifically, the invention relates to machinable synthetic bone substitute material implants having mechanical properties comparable to those of natural bone.
2. Summary of the Related Art
Treatments for bone voids, defects, and injuries must provide structural integrity and induce the formation of new bone. In particular, spinal fusion is designed to stabilize the spinal column by creating a bridge between adjacent vertebrae in the form of a bone fusion mass. Early spinal fusion methods involved stabilizing the spinal column with a metal plate or rod spanning the affected vertebrae and allowing bone fusion to occur around the implanted hardware. Various other forms of metal implants have also been used in spinal fusion procedures. However, the strength of metal implants causes stress shielding of the surrounding bone, which slows the natural bone growth that leads to fusion. Further, metal implants are permanent foreign bodies that cannot be remodeled into natural bone in vivo. In addition, many surgical procedures for implanting metal devices are long and complex.
Natural bone grafts have been used to promote osteogenesis and to avoid the disadvantages of metal implants. Naturally-occurring bone mineral is made of nanometer-sized, poorly-crystalline calcium phosphate of hydroxyapatite structure with a Ca/P ratio between 1.5 and 1.7. These properties impart solubility to bone tissue that allows it to be repaired continually by osteoclasts and osteoblasts. Natural bone grafts are incorporated into a patient's bone through this continual remodeling process in vivo. However, natural bone grafts are associated with problems such as limited availability and painful, risky harvesting procedures for a patient's own autogenous bone, and risks of viral transmission and immune reaction for allograft bone from a cadaver.
Synthetic bone graft materials have been used to avoid the problems associated with natural bone grafts. Desirable properties for synthetic bone graft materials include the following: chemical biocompatibility with natural bone; structural integrity, so that the graft remains in place and intact until bone heals around it; resorbability, so that the foreign material is replaced by bone and is accessible by osteoclasts, osteoblasts, and other bone-forming cells; and compatibility with low-temperature processing, which is required for incorporating heat-sensitive bone growth proteins to stimulate osteoblasts. Bioceramics have been used as bone graft substitute materials, providing a matrix that encourages new bone growth. Most commonly used have been the calcium phosphate ceramics hydroxyapatite and tricalcium phosphate. Hydroxyapatite is chemically similar to and biocompatible with natural bone. Highly crystalline hydroxyapatite has been produced that is dense, and therefore strong. However, such crystalline hydroxyapatite is essentially insoluble in vivo, and thus is not replaced by natural bone. Hydroxyapatite solids of lower crystallinity have been reported that are resorbable, but are not strong enough for spinal fusion applications or other applications requiring high-strength materials. Similarly, tricalcium phosphate materials generally are degraded rapidly in vivo, but lack sufficient strength for weight-bearing applications. Combinations of hydroxyapatite and tricalcium phosphate have been reported, which attempt to mitigate the shortcomings of the individual calcium phosphate components.
A ceramic implant of high strength and having the biological properties of natural bone, without the disadvantages of prior art materials, has proven elusive. Thus, a need remains in the art for bone substitute material implants that are biocompatible and resorbable, yet strong enough for use in applications requiring high strength, for example, in spinal fusion applications to support the spinal column until adjacent vertebrae have fused.